1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device suitable for an inverting amplifier.
2. Description of Related Art
An inverting amplifier is often used in a conventional analog circuit. For example, in a radio transceiver, inverting amplifiers are used in circuit blocks such as a local oscillation buffer circuit and a current-voltage conversion circuit, and frequency of use of inverting amplifiers is high.
As an example of such an inverting amplifier, a CMOS inverting amplifier configured to perform stable amplification at low voltage is disclosed in Japanese Patent Application Laid-Open Publication No. 2006-60606 (hereinafter referred to as Document 1). The inverting amplifier in Document 1 is a single-phase amplifier circuit. For the reason, the invention of Document 1 is disadvantageous, e.g., in that the inverting amplifier is susceptible to common mode noise, and output amplitude may become small or oscillation may occur due to in-phase components.
Accordingly, a differential amplifier circuit using a current source may be adopted as an inverting amplifier. In such an inverting amplifier, a common source of a pair of transistors with a differential configuration is connected to a current source, and a bias current is supplied by the current source.
An ideal current source as such a current source configured to supply a bias current is a current source whose output impedance is infinite and which is capable of supplying a constant current regardless of an output voltage. An actual current source is composed of, e.g., a transistor and uses, as a bias current, a drain current obtained by controlling a gate voltage. However, in a transistor, an output impedance decreases, and a drain current increases with a rise in output voltage, due to channel length modulation effect caused by miniaturization. A transistor thus cannot constitute an ideal current source.